1. Field of the Invention
The present invention relates to a particle concentration measuring device, more particularly, to a particle concentration measuring device capable of measuring particle concentration with high accuracy at a place in a clean room where a uniform unidirectional flow is ensured, or at other places.
2. Description of the Related Art
In a clean room and the like, measuring or managing concentration of particles such as dust floating in the air (the number of particles in a unit volume) is an important issue.
As this kind of conventional device measuring concentration of particles (particle concentration), the most popularly known one is a device of a sampling suction type that sucks the air through a sampling tube by the operation of a suction pump and finds the number of particles in the air by a light-scattering method. However, the need to install the sampling tube at a measurement target position makes this device unable to measure in a space where the flow changes due to the suction and thus a degree of freedom in its measurement place is low.
Further, in the measurement by this device, an airflow velocity and a suction speed are desirably equal to each other, but it is practically difficult to adjust the operation of the suction pump so as to make them equal to each other, and in addition, increasing the suction time in order to obtain a sufficient volume of the sampled air results in the detection of not only particle concentration at the target point but also particle concentration in a space far more upwind than this point, which has been a cause of deteriorating detection accuracy.
Another proposed device is a device measuring particle concentration from a remote place by a laser radar method. This device synchronizes a laser pulse and a shutter timing of a camera and measures the particle concentration by counting scattered lights of particles in a volume defined by a laser pulse width and a shutter speed.
The aforesaid device measuring the concentration from a remote place by the laser radar method is suitable for measuring the concentration of a large volume of particles in the atmosphere or the like from a remote place, but in an indoor space or the like, the laser pulse width and the shutter time become extremely short as a sampling volume becomes small, which makes it difficult to obtain light intensity large enough for the measurement, and thus the device is not practically capable of measuring small particles of 1 μm or less.
There have been also known an air impurity monitor which is capable of measuring impurity concentration in an atmosphere in a clean room automatically, speedily, and continuously and which is easy to handle and costs low (Japanese Patent Application Laid-open No. Hei 9-145616) and a dust measuring device used in a clean room and structured to receive dust by a dust collection sheet (Japanese Patent Application Laid-open No. 2003-42934).
The device of Japanese Patent Application Laid-open No. Hei 9-145616 sucks impurities to a hermetically closable impurity capturing vessel to supply the impurities into ultrapure water, and it not only has the same drawback as that of the aforesaid device sucking the air through the sampling tube but also has a complicated device structure. Further, the device of Japanese Patent Application Laid-open No. 2003-42934 is structured to receive the dust by the dust collection sheet, and is not suitable for measuring particle concentration (the number of particles in a unit volume).